I am using iTextsharp - a java pdf lib - to generate pdfs dynamically.
Now as I understood it, measurements are specified in points. I know where to place what on the place in cm.
so I need the conversion:
points <-> cm

Your assumption that measurements are specified in points is only partially correct but in your use case to generate pdfs dynamically that's good enough.

In general, though, measurements are specified in user space units, and the default user space unit (i.e. before any transformations are in place) can be configured on a per-page basis to be any float multiple (subject to implementation specific limitations) of 1/72 inch:

UserUnit
number
(Optional; PDF 1.6) A positive number that shall give the size of default user space units, in multiples of 1⁄72 inch. The range of supported values shall be implementation-dependent.